Safe operating area

For power semiconductor device (such as BJT, MOSFET, thyristor or IGBT) a safe operating area (SOA) is defined as the voltage and current conditions over which the device can be expected to operate without self-damage.

SOA is usually presented in transistor datasheets as a graph with V_CE (collector-emitter voltage) on the abscissa and I_CE (collector-emitter current) on the ordinate; the safe 'area' referring to the area under the curve. The SOA specification combines the various limitations of the device — maximum voltage, current, power, junction temperature, secondary breakdown — into one curve, allowing simplified design of protection circuitry.

Often, in addition to the continuous rating, separate SOA curves are provided for short duration conditions (1 ms pulse, 10 ms pulse etc.).

SOA specifications are incredibly useful to the design engineer working on power circuits such as amplifiers and power supplies as they allow him/her to quickly determine the conditions under which a design will catch fire, allowing the design of appropriate protection circuitry and/or selection of a more capable device. SOA curves are also important in the design of foldback circuits.

The most common form of SOA protection used with BJTs senses the collector-emitter current with a low-value series resistor; the voltage across this resistor is applied to a small auxiliary transistor that progressively 'steals' base current from the power device as it passes excess collector current.

This approach is effective but not bullet-proof. In practice it is very difficult to design a protection circuit that will work under all conditions, and it is left up to the design engineer to weigh the likely fault conditions against the complexity and cost of the protection.

Types of safe operating area

* Reverse bias safe operating area (or RBSOA) is SOA when turning the device into the off-state. The RBSOA may be different from generic SOA, for example for IGBTs, upper half corner of the RBSOA is progressively cut out which reduces the RBSOA as the rate of change of the collector to emitter voltage across the device, dVce/dt, is increased.

* Forward bias safe operating area (or FBSOA) is SOA when turning the device into the on-state.

See also

* Derating